1Field of the Invention
The present invention relates to a device for detecting longitudinal cracks on slabs, in particular on steel slabs issuing from a continuous casting.
2. Discussion of the Background
It is known that continuous casting creates difficulties due to the appearance of surface defects or cracks on the faces of the slabs. These defects must be absolutely repaired before the slabs are drawn and rolled.
The solution consisting in systematically retreating all the slabs is too expensive and it is therefore necessary to detect the cracks on the slabs at the output end of the continuous casting so as to deviate to a subsequent retreatment solely the slabs which have cracks requiring said treatment.
These cracks extend longitudinally relative to the axis of the slab and may be located just as well on the lateral faces as on the main faces of the slab.
A device for detecting cracks on slabs by means of eddy current probes is known and comprises an induction coil fed with alternating voltage and means for detecting variations in said voltage indicating the presence of a crack.
This device comprises on each side of the passage of the slabs a vertical probe maintained in proximity to an edge, and a plurality of horizontal probes located at different levels and maintained in proximity to a lateral face of the slab.
The presence of a crack substantially modifies the impedance of the induction coil and the eddy currents and results in a variation in the amplitude of the voltage exciting the coil and in the phase shift between the voltage and the current exciting said coil.
The analysis of the excitation voltage of the coil by electronic circuits provides an electric signal which presents amplitude peaks in the presence of a crack.
But this device detects solely cracks located on the narrow faces of the slabs and does not permit detecting the presence of longitudinal cracks on the main faces of the slabs